firefox: fix DeprecationWarning with firefox_options (#634)
firefox: fix DeprecationWarning with firefox_options (#634)

Fixes:

> DeprecationWarning: use options instead of firefox_options
cobrateam/splinter,bmcculley/splinter,bmcculley/splinter,cobrateam/splinter,cobrateam/splinter,bmcculley/splinter